To provide the ability to generate a restoring force to compensate for non-zero shaft eccentricity in a non-self-acting bearing, it is known that a restriction of the passageway from a supply pressure to the bearing surface, e.g. using a plurality of jets, is needed.
Each jet restricts the supply pressure to an intermediate pressure that is also dependent on the bearing gap width (i.e. distance between the bearing surface and the shaft) at the jet. The smaller the bearing gap width the higher the intermediate pressure. Under concentric conditions the intermediate pressures created by diametrically opposed jets is equal by symmetry. However, if the axis of the shaft is displaced from the axis of the bore (i.e. non-zero eccentricity) the intermediate pressure at a jet with a smaller bearing gap width will be higher than the intermediate pressure at a jet with a larger bearing gap width, thereby causing a restoring force which acts to correct the misalignment. If the passageways to the bearing surface are not restricted, the shaft experiences only the supply pressure; no pressure unbalance and therefore no restoring force occurs when there is non-zero eccentricity.
FIG. 1 is a cross-sectional view of a journal gas bearing 10 in which jets are provided to restrict gas passage between a gas supply and a bearing gap. The gas bearing 10 comprises a bearing housing 12 having a shaft bore 14 formed therein for receiving a shaft 16. The inside wall of the shaft bore 14 is a bearing surface 18. Radial passageways 20 through the bearing housing 12 provide fluid interaction between the bearing surface 18 and a pressurised gas supply (not shown). For example, as indicated by arrows in FIG. 1, compressed air may be pumped into each passageway 20 through a respective opening 22.
A jet 24 is inserted as a plug into each passageway at the entrance on the bearing surface 18. Each jet 24 is a cylindrical block sized to fit tightly into the passageway 20. Each jet 24 has a narrow through hole (e.g. rounded air gas-outlet) 26 which provides fluid communication from the bearing surface 18 to the interior of the passageway 20. The tight fit of the jet 24 in the passageway 20 ensures gas from the passageway 20 can only reach the bearing surface via the through hole 26.
As shown in an expanded view of a jet 24 in FIG. 1, the through hole 26 includes a constricted portion 28 located towards the bearing surface. The through hole 26 includes tapering section 30, 32 in which the diameter of the through hole decreases towards the constricted portion 28.
Each jet is made as a separate part, e.g. formed by a shaped drill or by a turning operation, before being mounted radially in the bearing housing 12.
However, as the desired operation speed of rotating machinery increases, so the size of components, e.g. shaft diameters and bearing gap widths, needs to be reduced. Such size reduction results in a need for very small jet diameters. As desired jet diameters approach 75 μm or less, e.g. 50 μm or less) the limit of what can be effectively machinable in a commercial manner draws near. In particular, any advantage associated with manufacturing jets as separate components is been replaced by problems resulting from assembling the jets in the bearing housing, finding suitable materials for the jets and finishing the bearing surface.
In light of these problems, it has been suggested that radially oriented laser-drilled blind holes may be used in place of jet plugs for high-speed bearings (e.g. for relative rotational speed in excess of 200 ms−1). U.S. Pat. No. 5,645,354 discloses an example of laser-drilled micro-holes formed in a bearing surface.
FIG. 2 is a cross-sectional view of a journal gas bearing 34 in which laser-drilled micro-holes are provided to restrict gas passage between a gas supply and a bearing surface. Components in common with the bearing shown in FIG. 1 are given the same reference numbers are not described again.
In this arrangement each passageway 20 terminates at the bearing surface 18 with an integral restriction 36. The expanded part of FIG. 2 shows the integral restriction 36 to comprise a radial blind hole 38 having a flat bottom 40 with a laser-drilled capillary 42 being the sole means for providing fluid communication between the bearing surface 18 and the radial blind hole 38. The capillary 42 is formed by directing a high energy beam, e.g. laser beam onto the flat bottom 40 of the blind hole 38 from outside the bearing housing 12. FIG. 2 shows the laser-drilled capillary 42 to have a nozzle-like shape, i.e. tapering to provide the smallest diameter at the bearing surface 18 as recommended by U.S. Pat. No. 5,645,354.
The intermediate pressure is predominately determined by the diameter of the laser-drilled capillaries.
The arrangement shown in FIG. 2 may help to address problems regarding material mating, deformation and make the production of journal bearings more cost effective.
US 2008/0256797 discloses laser-drilling capillaries from inside the shaft bore.